This invention relates to cementing in subterranean wells.
For over ninety years oil well casings and liners have been cemented with Portland cement. Portland cement has continued to be used during this time period in spite of the fact that it has two serious deficiencies. First, it is incompatible with most drilling fluids. Second, it tends to allow gas migration and gas leakage in situations where the wellbore goes through a gas producing formation. The term "gas migration" is used herein to mean the migration of gas and/or liquid. Similarly, the term "gas leakage" is used herein to mean the leakage of gas and/or liquid.
Gas migration through a cementitious slurry is the result of complex chemical and physical processes occurring after the cementitious slurry is placed in an annulus. Gas migration may occur at the formation/cementitious slurry interface, the cementitious slurry/casing interface or through the matrix of the cementitious slurry. This usually occurs within a few hours of the placement of the cementitious slurry in the annulus.
Gas leakage is flow of gas or liquid around or through the hardened cement sheath in the annulus. Gas leakage may occur at the casing/cement interface, the cement/formation interface, or through channels in the cement. Leakage of gas or liquid may occur as a result of (1) poor drilling fluid removal, (2) incomplete cement sheath around the casing, (3) casing contraction resulting in a micro annulus at the casing/cement interface, (4) shrinkage of the cement during hydration, (5) dehydration of the drilling fluid filter cake, (6) fluid loss to the formation during hardening or (7) gas migration through the cementitious slurry as it hardens thus leaving channels.
Cementitious slurries are suspensions of reactive or hydraulic solids in water. Chemical hydration of the cementitious component results in the transformation of the liquid slurry into a solid which is intended to provide a hydraulic seal in the annulus and provide support for the casing or liner. Hydration of the cementitious component occurs in stages and is accompanied by the liberation of heat and considerable gel strength development during the initial setting period.
Portland cement hydration begins immediately upon contact with water. The cement grains undergo a rapid period of hydration. Heat is liberated during this hydration, since most of the cement hydration reactions are exothermic. The surfaces of the cement grains become coated with a layer of initial hydration products. As this occurs, the water has less access to unhydrated portions of the cement grains, and the overall reaction rate decreases.
The decrease in reaction rate marks the beginning of the "dormant" or "induction" where essentially no additional hydration occurs. In a typical cementing operation, the cement is placed in the annulus during the dormant period. Gelation can occur during the dormant period from electrostatic and chemical interactions between cement particles and/or additives. Generally, gelation in Portland cement results in the creation of such high gel strength as to prevent transmission of sufficient hydrostatic pressure to hold back the gas or liquid through hydraulic pressure, but insufficient gel strength to physically hold back the gas or liquid. That is, the hydrostatic pressure of the cement slurry drops below the formation pressure while the cementitious slurry is not yet a solid. Hence, gas migration through the matrix.
Shrinkage occurs because the volume of a new reaction product formed during Portland cement hydration is less than the volume of the unhydrated cement grains. Shrinkage has little effect on the mechanical properties of the set cement.
Two types of shrinkage occur--bulk or physical shrinkage and internal shrinkage. Bulk shrinkage is the reduction in overall volume of the cement mass, i.e., dimensional decrease. Internal shrinkage is the reduction of the volume of cement paste in the cement matrix during hydration. Bulk shrinkage may be up to 7% by volume.
Bulk shrinkage results from the solids settling, filtrate loss, hydration of the cement and inner shrinkage. Inner shrinkage and bulk shrinkage both occur during the setting and early hardening periods of the cement. Bulk shrinkage may occur during the dormant period while inner shrinkage begins in the setting period. The total shrinkage rate increases during the setting period. Furthermore, some of the shrinkage occurs after the cement has lost its ability to transmit hydrostatic pressure. This can result in a significant pressure decrease in the cemented interval in the annulus.
The pores formed from internal shrinkage of the cement are characteristically free pores, which are interconnected and increase the permeability of the cement.
All of these problems are magnified in offshore drilling operations because of the problem of avoiding gas migration while installing subsea assembly devices on the ocean floor. Gas or liquid pressure is also a problem in instances where it is desired to plug a wellbore which traverses a zone of high formation pressure. For instance, when it is desired to plug a well in anticipation of a hurricane during offshore drilling, it is highly undesirable for the plug to allow passage of the gas or liquid.
Gas pressure can manifest itself in several ways. Generally, the term refers to the actual formation pressure. This can vary from essentially zero to as high as 10,000 psi or even greater. Generally, the zones of formation gas or liquid pressure encountered at greater depths exhibit higher pressure than zones encountered at more shallow depths, although, the inverse can be true in some cases. Pressures greater than 5,000 psi, generally at 5,000 to 9000 psi can be encountered in deep offshore wells.
The term gas pressure differential refers to the difference between the formation pressure and the hydrostatic pressure of the cement slurry column at a given location. The cement slurry hydrostatic pressure must be at least as great as the formation pressure or gas migration into the slurry will occur. Prior to the onset and gelation or hardening, the formation pressure is held back by the hydrostatic head of the cementitious slurry. However, as the hydrostatic head is reduced or not transmitted by this onset of gelation or hardening, the formation pressure becomes sufficient to overcome the hydrostatic head as it is reduced or not transmitted by this onset of gelation and hardening. As little as 0.1 psi greater pressure in the formation can cause gas migration. However, generally there is a mud or cement filter cake present in the formation, in which case the differential pressure will generally have to be 10-50 psi for gas migration to occur at that interface. Typically pressure differentials of 100-500 psi cause gas migration.
The third way gas pressure manifests itself is in creating gas pressure in the annulus surrounding the casing after the cement hardens. This results if there is gas migration which produces channels in the cementitious slurry as it hardens and/or at the interface of the casing or wellbore (gas leakage) or if the mass of cement is simply permeable to the gas. This gas pressure can range from low levels such as 50 psi to dangerously high levels of, say 1,000 to 5,000 psi or higher. This can collapse the casing, or otherwise cause a dangerous condition and is unacceptable.
Recently it has been discovered that, surprisingly, that blast furnace slag can be combined with drilling fluid to make a cement which is superior to Portland cement because of its compatibility with drilling fluids and thus avoids the incompatibility problems at the cementitious slurry/drilling fluid interface which are exhibited by Portland cement, Hale et al., U.S. Pat. No. 5,058,679 (Oct. 22, 1991).
It would be desirable to avoid gas migration problems in oil cementing operations in general. It would further be desirable to avoid gas migration problems during the setting of subsea assemblies in offshore drilling. It would also be desirable to allow plugging of wellbores traversing a zone of high formation pressure in a manner that does not allow passage of the gas or liquid.